This invention relates to an adsorbent catalyst for purifying exhaust gases or combustion gases, especially exhaust gases from gasoline, diesel or natural gas engines.
With gasoline engines, significant decreases (10-20%) in fuel consumption have been attained with new engines where direct injection of gasoline and excess air are used. Depending on running stage, the blend ratio air/fuel (A/F=air/fuel) is in these engines clearly on the dilute side (xcex is between 1.1 and 2.4) for extended periods, whereby air is introduced in excess in relation to the amount needed for complete combustion of the fuel. At other times, the blend ratio of the engine is stoichiometric (xcex=1) or rich (xcex less than 1), whereby these circumstances must normally be adapted to in varying driving conditions and by high driving speeds. The management system (EMS=Engine Management System) strives to comply with the running parameters programmed in the memories of the vehicle that are based on the engine and driving condition mapping of the automobile maker. A diesel engine is traditionally more fuel conscious especially in heavy motor vehicles, and the injection technique of the fuel has developed together with the engine management systems to the point, where raw emissions and especially the amount of emitted particles are minor. The increasing change-over to dilute blend engines has, however, made the functioning of the traditional removal techniques of the nitrogen oxide exhausts more difficult.
With a three-function catalyst the concentrations of nitrogen oxides, carbon monoxide and hydrocarbons may be decreased by over 90% efficiency during normal motor vehicle test cycles (according to Euro 3/Euro 4 levels). When using a three function catalyst, one strives to keep the circumstances (air/fuel blend ratio, A/F) as stoichiometric as possible with the aid of responses from the xcex sensors and the engine management system. The emission limits for particles and for NOx become stricter when shifting from Euro 3 to Euro 4: particles down to 0.025 g/km and Nox down to 0.25 g/km.
However, in exhaust gases containing oxygen in excess, the amounts of reducing agents and their selectivity towards reducing NOx are low. Normally, gaseous emissions (HC, CO) are purified at more than 80% conversion, and particles at about from 20% to 40% conversion when using an able oxidizing catalyst and xcex being clearly over 1. The conversion of nitrogen oxides is in diesel exhausts normally about from 0% to 20%, whereby even a four-function catalyst is mentioned (CO, HC, NOx, particles). In diesel oxidizing catalysts, normally Pt is used as the active metal, it being resistant to SOx. The catalysts are often positioned as close to the engine as possible (CC=Closed Coupled), instead of being positioned under the body (UF=Under Floor), as usual.
In order to decrease the amount of nitrogen oxides, Ir catalysts, used together with three-function catalysts or Pt catalysts, have been developed and used commercially even in Euro 2 level direct injection automobiles (GDI=Gasoline Direct Injection). However, this kind of catalyst constantly requires a relatively high hydrocarbon content in the exhaust gas, the thermal and chemical resistance of the Ir catalysts is rather poor (max 800-900xc2x0 C.), and the NOx conversion remains rather low. Even Ptxe2x80x94Irxe2x80x94Rh/ZSM5 catalysts have previously been used in dilute gasoline subjects (Iwakuni et al., Science and Tech. in Catal. 1998). Although one has strived to increase the endurance of zeolite based catalysts, it is, however, insufficient in full load driving circumstances.
Because catalysts based on continuous NOx reduction do not function at the levels required by Euro3 or Euro4 exhaust limits, new types of catalyst have been developed that are based on NOx adsorption (NOx trap) (SAE Publication 982593). Their functioning is based on adsorbing nitrogen oxides during dilute phases into the NOx trap components of the catalyst, one of which is e.g. BaO. Nitrogen oxides are oxidized to NO2, they are adsorbed as nitrates (Ba(NO3)2) in the dilute phase (duration, e.g. from 15 s to 5 min) and they are reduced to nitrogen in the short enrichment period arranged on purpose (duration, e.g. from 1 s to 5 s, A/F ratio less than stoichiometric). The method may be tuned (timing, regenerations) to function in accordance with the engine and the driving conditions. The most usual adsorbent Ba binds the sulfur oxides (SOx) of the exhaust gas tightly as sulfates on its surface, and this weakens functioning of the catalyst as time goes on. Depending on the sulfur content of the fuel, the catalyst must be totally regenerated every 1000-6000 km to rid it of sulfate. Barium (Ba) sulfate can be decomposed by raising the temperature of the exhaust gas to more than 650xc2x0 C. for a time long enough in clearly rich circumstances. Thereby the sulfates are decomposed as sulfides, and are liberated mainly as hydrogen sulfide or COS into to exiting exhaust gas. Thus the functioning of the catalyst is partly or totally recovered. In unfavorable circumstances, BaO may bind CO2 as carbonates on its surface more tightly than nitrates. The functioning of catalysts based on Ba calls for a very low sulfur level of the fuel, and also, the level of functioning of the catalysts mentioned is not high enough.
The exhaust gas flows continuously through the NOx trap catalyst, the continued functioning of which is based on the enrichments guided by the management system (EP 0 560 991, 1993). HC and CO are presented as functioning as reducing agents in the enrichments and the dilute phases are of a duration more than 50 times longer than the enrichment phases. The NOx adsorbent in the catalyst is K, Na, Li, Cs, Ba, Ca, La or Y and as noble metal, Pt. Also a Baxe2x80x94Cu composite is claimed to function. After the NOx trap catalyst, or before it, there may be a normal three-function catalyst. In the EP patent, even a system for purifying diesel engine exhausts with the same catalyst is described.
Another concept (EP 0 838 255) contains on the surface of a porous support (aluminum oxide) alkali metal (Na, Li, K, Rb), Ti and a noble metal (Rh, Pt, Pd). Also, the catalyst may contains rare earth metals or Mg. It has been claimed that by very high Na and K contents (over 30% by weight, EP 0 857 510) or Li contents (over 10% by weight, WO 97/47 373) together with noble metals, an efficient NOx trap material has been accomplished. Sr (6-15% by weight) may be used together with Zr and sulfate even in place of Ba in the catalyst (U.S. Pat. No. 5,753,192).
With various mixed oxides (in general AaBbO4, where B is mainly Al, and A is Mg, Ca, Mn, Fe, Ni, Co, Cu, Zn, Sn or Ti) there have been accomplished NOx adsorption capacities (WO 98/56 492).
Use of W-phosphate in the Pt catalyst has been proposed in order to hinder the harmful effects of sulfur in NOx adsorption (EP 0 864 353).
A solution has been proposed, according to which there are two NOx trap catalysts side-by-side whereby concomitantly one is at the adsorptive stage and the other at the reductive stage (WO 98/45 582). The catalyst contains Sr, Ba, Ca, BaTiO3, BaZrO3, LaZrO3, MnO2, LaMNOx and a mixture of oxides of La, Ce, Ti and Zr, as well as Pt, Rh or Pd. Similar compounds (BaMnO3, CaMnO3) have been used also in another Patent (U.S. Pat. No. 5,906,958). The method is complicated and it requires a double amount of the catalyst as well as a three-way valve that functions in hot circumstances (even 900xc2x0 C.), in order to direct the exhaust gas.
It has been claimed that a mixture of Ti/mordenite and mixed oxide (La, Ba, Co) functions in the presence of hydrocarbons, giving a good NOx conversion (DE 44 45 945).
There has been presented a two-layer catalyst where the Pt and NOx adsorbents (Ba, K) are in the first layer and the Rh is in the second layer (WO 99/00 177). The aim is to oxidize NO to NO2 in the Pt layer and to reduce the nitrogen oxides liberated to nitrogen in the Rh layer. Also, it is proposed that a stoichiometric blend ratio is sufficient in order to regenerate (reduce) the nitrates of the catalyst surface. Even in a former publication (WO 97/02 886) there has been proposed a two-layer catalyst in which in one layer there are the NOx trap material and Pt, and in the other, the reduction catalyst. The layers may even be blended into each other. Usually, in a two-layer catalyst separate compositions disturb one another (noble metals pass in adsorption into the same layer, addition of certain NOx trap compounds (Ba, K) disturbs the regeneration of nitrates=flammability problem). The decomposition and reaction of the nitrates can not begin before the active site in question starts to function in the reaction during the enrichment peaks. It is difficult to make function a NOx trap catalyst in which in the same layer there have been added compounds that bind nitrates at both high and low temperatures, because they interfere with each other, whereby usually the higher temperature NOx trap capacity is dominant.
In one solution (EP 0 778 072) three catalysts have been mounted is series: the first is the dilute side NOx catalyst, the second is the NOx trap and the third is a three-function catalyst. However, the solution is complicated and the efficiency of known NOx catalysts is poor due to the low HC content and probably, it does not function well during enrichments peaks.
It has been proposed that the harmful effects of sulfur be decreased by using in front of the NOx trap catalyst an adsorbent unit for sulfate (EP 0 892 159). By this kind of solution, usually, only a slow-down of the deactivation of the NOx trap catalyst situated behind the former by sulfates may be accomplished, because in rich mixture peaks gaseous sulfur compounds should pass through the NOx trap catalyst without adsorbing thereon. Thereby the regeneration of the NOx trap catalyst (desorption of sulfides) in rich phases is, in any case, the decisive characteristic in view of a prolonged use.
There has been proposed even a catalyst solution where the surfaces of two metal foils have been coated with catalyst surfaces, the support material, physical structure or noble metals of which are mutually distinct in various locations (FI 94 455). In this way, good results may be gained in chain or serial reactions that cannot be attained in the same catalyst layer. However, the method is meant to be used in context of a pure catalyst reaction, the use of adsorptive methods together with the catalyst in circumstances of varying composition changes the situation altogether.
The catalyst reaction is based on continuously progressing stages, where reactants and reaction products participate: 1) the outward transfer of matter to the outer surface of the catalyst and away from the surface, 2) pore diffusions from the outer surface to active sites in the pores and away, 3) adsorptions and desorptions to active sites, 4) surface reactions at active sites. Although adsorption is one of the stages in the mechanism of the catalyst reaction, the adsorption of reactants may proceed only to a certain equilibrium, after which more of the component in question cannot be adsorbed onto the catalyst. Too much adsorption is usually the reason for that reactions will not start at low temperatures. The adsorptive methods per se are totally distinct from the catalyst methods, and they aim at storing certain compounds onto the adsorbent, which may e.g. be changed from time to time to a new one, or be regenerated after the adsorptive capacity has been exhausted. In catalysis and in adsorption the chemical nature of active sites is also very different. However, in exhaust gases the amount of impurities to be removed, such as the NOx""s, is so high that the adsorptive capacity is, depending on the circumstances, exhausted in a few minutes, at the latest.
This invention aims to provide a continuously functioning combination of an adsorbent and a catalyst suited for NOx removal as well as a method with the aid of which high conversion levels of nitrogen oxides to nitrogen are attained, when the exhaust gas blend contains, on the average, an excess of oxygen and typical impurities from fuel and lubricant and the engine.
The invention is based on the fact that the composition, structure, and method, as well as circumstances of making the adsorbent catalyst, are such that a clearly distinct and better solution is obtained compared to the catalysts previously proposed, the resistance and regenerability of which in the presence of sulfur dioxide have also improved.
Inventive fields of use are exhaust gas and combustion gas applications in moving and stationary subjects, where the combustion ratio of the blend is dilute, that is to say, containing excess oxygen continuously or temporarily, and as fuel, any liquid, gaseous or solid fuel may be used.
The adsorbent catalyst concept according to the invention comprises several different chemical compounds, the function of which is, in one or several catalyst reactors, to catalyze oxidization of NO to NO2, to adsorb nitrogen oxides/nitrates/nitrites and to catalyze reduction of nitrogen oxides to nitrogen, to adsorb reducing agents by rich blends, to catalyze oxidization of CO/hydrocarbons/hydrogen to water and carbon dioxide even in circumstances containing sulfur compounds.
According to the invention, there is thus provided an adsorbent catalyst for reducing the amounts of nitrogen oxides, hydrocarbons and carbon monoxide contained in exhaust or combustion gases, which catalyst adsorbs nitrogen oxides, when the exhaust or combustion gases contain excess oxygen, and liberates and reduces the adsorbed nitrogen oxides, when the gases mentioned contain oxygen in stoichiometric amounts or less, which adsorbent catalyst is characterized in that it comprises a porous support material the surface area of which is large and which contains at least the following:
a first catalytic metal, which is preferably Pt, a first NOx adsorbent, which preferably contains at least one of the following metals: Ba and Sr, a second NOx adsorbent, which preferably contains at least one of the following metals: La and Y, and a redox NOx adsorbent, which preferably contains at least one of the following metals: Ce, Zr, Ti, Nb, Mn, Pr, Nd, Sm, Eu and Gd.
The support material may additionally contain a second catalytic metal, which preferably comprises at least one of the following metals: Rh, Pd and Ir.
The support material may additionally contain a third NOx adsorbent, which preferably contains at least one of the following metals: K, Na, Li, Ca, Tb and Cs.
The support material may additionally contain a fourth NOx adsorbent, which preferably contains at least one of the following metals: Mg and Be.
The adsorbents mentioned may be in the form of oxides, sulfates, nitrates, aluminates or as metals, preferably they are in the form of oxides.
The redox NOx adsorbent mentioned preferably contains preferably Ce and/or Zr.
The redox NOx adsorbents mentioned preferably have formed between them a mixed oxide or a mixture thereof.
Preferred redox NOx adsorbents are for example mixed oxides of ZrCe, MnCeZr and MnCe.
Preferably, of the redox NOx adsorbent mentioned, and of the second NOx adsorbent, a mixed oxide comprising two or more elements or a mixture thereof has been formed.
The average particle size of the first and/or the second catalytic metal is preferably less than 30 nm in fresh support material, particularly preferred is less than 10 nm.
The average particle size of the redox NOx adsorbent is preferably more than 5 mm in fresh support material, particularly preferred is from 5 mm to 30 mm.
An adsorbent catalyst according to the invention may be structured such that it comprises a first surface, on which there is a first coating that contains the support material and said adsorbents, or a part thereof, and a second surface, on which there is a second coating, which contains the support material and said adsorbents, or a part thereof, whereby the compositions of the coatings are identical or varied.
The first surface mentioned may comprise an essentially smooth metal foil, and the second surface may comprise a corrugated metal foil, whereby the metal foils form a honeycomb that has numerous flow channels for gas.
According to one embodiment both coatings comprise the first catalytic metal Pt, while only the second coating comprises a second catalytic metal.
Preferably both sides of the metal foils mentioned are coated.
Preferably the first catalytic metal has been divided concentration wise between various foils in such a way that in one foil the Pt load is about 0-90 g/ft3, and in the other foil the Pt load is about 70-400 g/ft3, whereby the volume relates to the volume of the honeycomb formed by the foils. Here the Pt load in one foil may be about 10-90 g/ft3 and in the other about 70-250 g/ft3.
The first NOx adsorbent may be divided concentration-wise between various foils in such a way that in the support material of one foil the concentration is about 8-40% by weight, preferably about 10-20% by weight, and in the support material of the other foil the concentration is about 0-10% by weight, preferably about 3-8% by weight, which percentages have been calculated as oxides of the weight of the support material.
The second NOx adsorbent may be divided concentration-wise between various foils in such a way that in the support material of one foil the concentration is about 8-40% by weight, preferably about 5-15% by weight, and in the support material of the other foil the concentration is about 0-8% by weight, preferably about 1-6% by weight, which percentages have been calculated as oxides of the weight of the support material.
The redox adsorbent mentioned may be divided concentration-wise between various foils in such a way that in the support material of one foil the concentration is about 10-60% by weight, for example about 10-30% by weight, preferably about 15-25% by weight, and in the support material of the other foil the concentration is about 0-10% by weight, preferably about 2-5% by weight, which percentages have been calculated as oxides of the weight of the support material.
It is even possible to form such a structure where the adsorbents are exclusively in the support material of one foil, which foil is preferably the smooth one, and catalytic metals are in the support material of the other foil, which is preferably the corrugated foil.
Preferred support materials are materials, which contain mainly at least one of the following oxides: aluminum oxide, zeolite, aluminum silicate and silica.
According to the invention we have additionally realized a catalyst system for reducing the amounts of nitrogen oxides, hydrocarbons and carbon monoxide contained in exhaust or combustion gases, which system contains the adsorbent catalyst described above and another catalyst positioned in front of the adsorbent catalyst in the direction of gas flow. This other catalyst may be e.g. a three-functional catalyst or a oxidizing catalyst. This other catalyst, in particular, oxidizes efficiently especially hydrocarbons, whereby a mixture is formed, in which the amount of hydrocarbons is low and during rich peaks the percentage of carbon monoxide and hydrogen is as high as possible before the adsorbent catalyst.
The other catalyst mentioned contains one or more catalytic metals, which is preferably one of the following metals: Pd, Rh and Pt.
According to the invention there is also provided a method for reducing the amounts of nitrogen oxides, hydrocarbons and carbon monoxide contained in exhaust or combustion gases, in which method the gases to be purified are introduced through the adsorbent catalyst according to the invention described above, or through the catalyst system according to the invention described above, which adsorbent catalyst adsorbs nitrogen oxides, when combustion or exhaust gases contain in excess of oxygen, and liberates and reduces the nitrogen oxides adsorbed, when the gases mentioned contain a stoichiometric amount of oxygen or less.
Short guided or natural periods may be used, where gases contain a stoichiometric amount of oxygen or less.
According to the invention there is further provided a method for adsorbing an ingredient in a medium onto an adsorbent catalyst and for reacting it catalytically with a reactant into a desired compound, which adsorbent catalyst contains one or more catalytic metals as well as one or more absorbents, and in which method the continuous functioning of adsorption and the catalyzed reaction are realized with the aid of heterogenous conditions.
In this method adsorption and a catalyzed reaction are united without them interfering with each other. The continuous functioning of adsorption and the catalyzed reaction may be provided with heterogenous conditions by proceeding in relation to a certain reaction on one side of the stoichiometric ratio in the adsorption phase and on the other side thereof in the catalyzed phase.
Heterogenous conditions may be provided with changes in temperature or pressure.
The medium mentioned may be a gaseous medium, such as exhaust gas or combustion gas, but even other media can come into question. Said ingredient can be e.g. a nitrogen oxide, whereby the reactant is carbon monoxide, hydrogen gas and/or hydrocarbon, and the product is nitrogen gas. Said ingredient can also be a hydrocarbon, whereby the reactant is gaseous oxygen and the products are carbon dioxide and water. Said ingredient can also be carbon monoxide, whereby the reactant is gaseous oxygen and the product is carbon dioxide. In addition, said ingredient can be an oxide of sulfur, whereby the reactant is hydrogen gas and/or carbon monoxide and the product is sulfur dioxide and/or hydrogen sulfide.
Preferably, long adsorption phases are utilized, whereby one or more reactants are from time to time brought to the adsorbent catalyst.
In this method, the adsorbent catalyst is preferably formed of an essentially smooth metal foil and of a corrugated metal foil, whereby one of the foils contains adsorbents or support material and adsorbents and the other contains a catalytic metal or support material and a catalytic metal.